1. Field of the Invention
This invention relates to a device for jetting a liquid with stability from an initial jetting stage and an ink-jet recording apparatus. More particularly, it enables prevention of second-operation liquid jetting failure by introducing a bubble into a liquid chamber. The present invention can be applied to an apparatus and a method for printing out information input into an office automation apparatus such as a personal computer or a word processor.
2. Related Background Art
In ink-jet recording based on jetting liquid droplets, there is a problem that after a long resting period or after turning on the main switch, a first ejection recording operation can be suitably performed but the performance of a second ejection recording is unstable.
It is known that if an electromechanical conversion member is used as an ejection element, such a problem is caused by a phenomenon in which high-frequency vibration of ink due to mechanical vibration moves an ink meniscus to and fro, or by the influence of pressure waves reflected by walls traveling toward the ejection holes. To solve this problem, various means have been invented which relate to improvements in an electrical signal applied to the electrothermal conversion member. However, this problem entails further complicated phenomena according to changes in ink characteristics and changes in environmental factors and, therefore, no method is known which solves this problem with reliability.
If an electrothermal conversion member is used as an ejection element, the change in recording performance is small while the corresponding change in the case of a mechanical conversion member is large. However, a method is known in which the influence of interference between an electrothermal conversion member and an adjacent ejection section is recognized to set the liquid passage length and the position of a liquid chamber wall in accordance with predetermined conditions. Specifically, Japanese Patent Laid-Open Publication No.55-128465 discloses an arrangement in which a small hole communicating with the atmosphere is formed in an ink passage wall on the liquid chamber side of the electrothermal conversion element to reduce the magnitude of a pressure wave. This arrangement is effective in practice but also entails the problem of ink evaporation through the small hole or the problem of ink setting and attaching in the vicinity of the small hole if the recording head is left unused for a long time, and there is also a possibility of operation failure when recording is started after the non-use period.
On the other hand, it is well known that a suitable initial ejection condition can be achieved by performing a suction recovery operation. U.K. Unexamined Patent Publication No.2184066 discloses an arrangement in which suction recovery is effected after introducing air into a common liquid chamber communicating with a plurality of ejection holes as well as into these ejection holes. Recovery in the common liquid chamber can thereby be effected at one time, which is advantageous.
In the field of conventional recording heads, however, no suitable function has been achieved which effectively prevents occurrence of recording failures immediately after the start of recording after a long unused period without being seriously influenced by environmental changes or various changes in conditions. With respect to other types of liquid jetting, there is no satisfactory method or apparatus for solving the problem.
In many of the conventional ink jet recording apparatuses, ink is continuously effected through one ejection hole depending upon recording data on an image or a character to be recorded. If such continuous ejection is effected after an unused period or immediately after the main switch has been turned on, a dot formed on a recording sheet by a second ejection operation for this continuous ejection is formed as a defective dot, usually resulting in a deterioration in recording quality. The cause of such failure to form a suitable dot by the second ejection operation has been studied to develop the following theory.
The force with which the ink flows towards the discharge end is given as follows: EQU &lt;capillary force&gt;+&lt;inertia force&gt;&lt;bubbling energy at liquid chamber side&gt;&lt;tank vacuum&gt; (1)
The refill time is determined in accordance with the force which is given by the formula (1) shown above.
It is to be noted, however, that the condition of the formula (1) above is not met when the discharge is performed for the first time. This is because the bubble is generated while the ink is in a stationary state. Namely, when the ink discharge is performed for the first time, the bubble is generated under the condition where there is no flow of ink towards the discharge end. Therefore, the force of flow of ink for refilling conducted between successive first and second discharges is given as follows: EQU &lt;capillary force&gt;-&lt;bubbling energy at liquid chamber side&gt;-&lt;tank vacuum&gt; (2)
As will be seen from the formulae (1) and (2) above, the ink supply characteristic for supplying ink from the common liquid chamber 1 to the nozzle, i.e., the force for forcing the ink towards the discharge end, is such that a longer refilling time is required in case of the formula (2), because the force for forcing the ink towards the discharge end is smaller in the case of the formula (2) than in the case of the formula (1). That is, a longer refilling time is required for the refilling after the first discharge than for other refilling operations. Usually, the time interval between the first and second discharges is not set specifically longer than other intervals. The ink droplet formation failure in the second discharge is considered to be attributable to this fact.
This causes a practical problem such that, when a vertical line is to be formed by two dots as shown in FIG. 8A, a line image in the form of an aggregation of tiny droplets is formed as shown in FIG. 8B, due to failure in the formation of the ink droplet for the second dot.
Anyway, if the first driving energy is applied to the ink having an inertia, an excessively large pressure is generated in the common liquid chamber, when the ink is discharged by energy of bubble produced by the electrothermal conversion element. Consequently, the ink is compressed by this pressure so as to have a large inertia. Meanwhile, the meniscus continues to move from the discharge opening towards the electrothermal conversion element until the ink is supplied. This tendency is maintained until the bubble is generated. In this state, the supply of the ink into the common liquid chamber tends to be delayed as compared with the case of ordinary recording operation, due to the large inertia applied to the ink in the common liquid chamber. In addition, the recording system relying on the ink discharge by means of the bubble exhibits a high speed of response to the recording signal. Consequently,the second driving signal is undesirably input to the electrothermal conversion element, so that the meniscus tends to further move towards the common liquid chamber. This also is attributable to the delay of propagation of the pressure generated by the bubble towards the common liquid chamber, due to the fact that the impedance of the flow passage between the bubble and the common liquid chamber is smaller than that between the bubble and the discharge opening. Thus, the filling of the common liquid chamber with ink is delayed and, in addition, retraction of the meniscus towards the common liquid chamber is increased. The discharge failure in the second discharge is attributable to these facts.
The bubble generated for the second discharge is more liable to move towards the common liquid chamber than in other discharges, partly because of the retraction of the meniscus. The state of extinction of the bubble also is different from that in other discharges.
Thus, it has proved that the novel technical subject described above, peculiar to the system which relies upon thermal energy for generating bubble, is entirely different from the problems which have been conventionally recognized in regard to the phenomenon or behavior of conventional electrothermal conversion elements.